In recent years drilling techniques for oil and gas wells have developed to the point where sophisticated controls may be used to alter the direction or rate of advance of the drilling bit. Among these techniques is the use of a device widely known as an adjustable stabilizer whereby a section of drill pipe (often referred to as a sub) near the bottom of the drill string slightly behind the bit, may be used to alter the manner in which the drill string rests against the side of the hole and thereby change the direction of the nearby bit, and hence the direction of the hole. These adjustable stabilizers frequently take the form of pads mounted in the sub which can be expanded radially to increase the diameter of the sub or released and contracted to narrow the effective diameter of the drill string at the location of the stabilizer sub.
In addition, the marginal profitability of many oil and gas well drilling operations requires that these control functions be capable of operating with a minimum of down time. It is, therefore, important to develop activating mechanisms which can be controlled by the driller without withdrawing the drill string and stabilizer sub from the hole which is a time consuming interruption in the drilling performance. Consequently, various techniques have been developed for manipulating the mechanism of adjustable stabilizers while the equipment is down hole.
Some of these prior mechanisms have been controlled by adjusting the weight on the drill string but this approach has the disadvantage that often it is very hard to determine the actual weight which is being applied at the down hole location of the stabilizer sub because deep, deviated holes have a large amount of "hole drag".
Other mechanisms are designed to be set or released by the flow rate at which drilling mud is pumped through the central bore of the drill string. However, often the rig is operating at close to the maximum flow rate and therefore there is little flexibility available to increase the flow enough to activate a tool of this design.
Still other mechanisms rely on controls activated by the pressure of the drilling mud but like the former flow set tools, a range of pressures is not always available to the driller, or it may not be desirable to operate the drill at less than maximum pressure. For the above reasons, it is important to develop mechanisms by which down hole devices such as stabilizers may be engaged or disengaged by operations which can be conducted at the drill floor but without limiting the drillers ability to select the desirable weight, flow rate or mud pump pressure for optimum conditions during drilling operations.